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Fuel mixture changes rich and lean every 7 seconds. GRAPH INCLUDeD
Car is a 1981 911SC
Had some problems with the CIS fuel injection recently. The fuel injection system was shutting on and off on me and then it was just shut down. Well, that problem was caused by the fuse for the interior lights. Now the fuel injection goes rich for 7 seconds and then it goes back to normal(lean) for 7 seconds. Repeating. See graph below from my data logger.
I checked the fuel injection pressures and they are in spec. The system pressure has some flutter in it but it does not cycle every 7 seconds with the fuel injection. I looked for a vacuum leak. Duty cycle stays stable and the o2 sensor is disconnected as it was picking up interference somehow.
Relay? It has a frequency valve and that could be the issue but I do not know how to diagnose it as such. The 02 sensor is disconnected at the ECU connection. That too was causing bizarre fuel mixtures. Most likely need another sensor but it seemed like the wire was picking up signal some how.
I checked the spark on all the cylinders at the distributor cap today. I thought that it might be a miss causing the change in air-fuel-ratio.
That would make sense as you have unburned fuel in there. I will post a pic of my LM-1 output as reference for you to compare to. The spike is where I blipped the throttle to see how fast it came back ..
This is from a 78sc so no lambda at all ..
somewhat odd yours is so cyclic , almost like it is constantly trying to correct for something.
maybe you are right . i thought there was some type of relay that had to be reset or something when you changed the o2 sensor ..maybe I dreamed it up
Ice, there's the frequency valve and the 02 sensor relay, not the frequency relay. The 02 sensor light needs to be reset but that is just a light that runs off the odometer every 30,000 miles.
I really like your graph. Could you disconnect one of your spark plug wires and remake the same graph? It is suppose to show a miss but it did not work for me when I tried it.
The narrow band O2 sensors only detect rich or lean. The system will cycle back and forth to create an average, but it isn't any where near as precise as modern systems. My own (1982 SC) will cycle as well. However, my cycling is much narrower (never leaves 14.1 - 14.9) and is more frequent than 7 seconds. More like the 3 seconds referenced in the table here: http://jimsbasementworkshop.com/CIS/...%202%20TT.html
I ended up pulling the WR5DC spark plugs and 4 of them were black (cylinders 2, 4, 5, & 6). I installed a set of the misguided Bentley Manual WR7DC plugs that I had laying around. I figured the heat would help with the fouling. The engine runs a lot better but still has some of the cycling of the fuel mixture (missing?). I decided to clean the advance in the ignition distributor since it seemed to be an ignition issue. It was pretty clean but I figured it could have been a little lazy. It did not end up making any noticeable difference. I can change the cap and rotor or maybe the wires or pull the injectors to clean but that was done 7 years ago.
Should I just wait a while and then check the plugs again after a little more use and change back to a set of WR5DC?
I pulled the plugs today and the fouling condition looked about the same as the last set. I checked compression on just 3 or 4 cylinders and cylinder #5 had 120 psi and cylinder #6 had 25 psi. The other cylinder/s were 185. This was disheartening enough that when a nice gentleman ran me down to offer and buy my car during the test drive, I actually gave him my number. I did a valve adjustment last summer and I am going to go back and double check that. I am also aware that the carbon could be giving me the low numbers. I could not see #6 being so low without a blown head gasket or a valve not closing completely. This is not a good time for me to try and rebuild an engine.
Yes, do check valve lash. A intake valve not properly closing will cause very erratic AFR numbers since it introduces tons of EGR into the intake track.
I also suggest finding the exact cause for the low compression. Hot gasses escaping through a valve at heavy engine loads in the power stroke can super heat a valve, to the point that the valve head will be damaged and even possibly come off the valve stem! Do not ignore this problem.
Leak down was 20% on #6. I ran about half a gallon of water through the engine to steam clean and it changed nothing. I checked the valve lash today and it was correct. I opened it up a little more and there was no change. The only thing I have left is to try a concentrated Techron fuel mixture for about an hour at lots of different loads. That is with the hope that it is just a carbon problem. If that does not work then it sits for 6 months or more till I can pull the motor or someone buys it.
Cyl 6 likely has:
- A very bad valve or valve seat, possibly even a cracked valve face.
- Terribly worn valve guides
- Could have cracked rings
- Compromised head to cyl seal
Some of the above conditions could have been from the result of lean mixture seen in the #6 cyl under load, resulting in detonation and cyl damage that will not be repairable without engine removal.
Hate to bring the bad news, but if the valve lash is good and you still have less than 50PSI compression that motor needs to be torn down to figure out why. A leak down test may held identify what's leaking, you can often hear a leak of that size via the intake or exhaust if it's a valve. If you do not hear the air in exh or int then you may hear it within the motor via the oil cap. Or by listening at the area near the head and cyl barrel. You could do one more in car test via the leakdown testing to help pinpoint the issue.
Hopefully you checked that the CIS Lambda ECU's signal to the frequency value
is not changing via an o-scope. Better yet, you can disconnect the Lambda ECU
and re-adjust (richen) the mixture temporarily. That will eliminate any Lambda
CIS interactions with the changing mixture.
09-04-2015, 11:37 PM
Fuel mixture changes rich and lean every 7 seconds. GRAPH INCLUDeD
The 02 sensor light needs to be reset but that is just a light that runs off the odometer every 30,000 miles. 
